Many electronic systems require protection from unexpected interruption of power. Supercapacitors have become widely used for applications requiring energy storage. Compared to conventional capacitors, supercapacitors provide superior energy density. Compared to battery cells, supercapacitors can often provide larger peak discharge currents and may be fully discharged without damage. One limitation of supercapacitors is their maximum rated voltage, which is typically 2.7 V. Many applications require voltages higher than 2.7 V, e.g. 12 V to 24 V. To meet this requirement, a bank of supercapacitors may be formed by connecting a number of supercapacitors in series. If N supercapacitors are connected in series, the rated voltage of the bank is N×2.7 V. For example, if N=5, a bank of five supercapacitors has a rating of 13.5 V. As is the case with the parameters of any electronic component, the actual capacitance of a supercapacitor has a manufacturing tolerance. For a supercapacitor, the tolerance of the capacitance is typically ±10% or ±20%. Thus, for a supercapacitor nominally specified to be 100 F, ±20%, the actual capacitance may be any value between 80 F and 120 F. This tolerance is significant with regard to charging and/or discharging a bank of supercapacitors connected in series. In common practice, a single charging circuit is used to supply current into the top (most positive) supercapacitor in the bank. Because the supercapacitors are in series, the same current flows into all of the supercapacitors, causing them to charge. Ideally, the charging process is described by the following equation:V=It/C  (1)where V is the voltage on each supercapacitor, I is the charging current, assumed to be constant, t is the elapsed time, and C is the capacitance.
Therefore, in a series string, if all of the capacitances are equal, all of the capacitor voltages are likewise equal. However, in practice, this fortuitous circumstance is precluded by the mismatch of the capacitance values due to the manufacturing tolerances. If, for example, one supercapacitor has a value that is 20% high and another supercapacitor has a value that is 20% low, the voltage difference at any time during the charging cycle is 40%. This may pose a problem if the charging process is controlled by measuring the total voltage across the series bank, with the charging terminated when a predetermined limit is reached. For example, consider a bank of five capacitors to be charged to 13.5 V. If the charging is controlled simply by checking the total voltage, and terminated when this voltage reaches 13.5 V, it is very likely that some of the series capacitors may have voltages less than 2.7 V, and other may have voltages greater than 2.7 V, although the average may be equal to 2.7 V. This is a very serious problem, because the individual supercapacitors are said to be “not forgiving,” in the sense that they are likely to be irreparably damaged by voltages exceeding 2.7 V. This issue is known and several precautionary steps are commonly used to address the problem. For example, the charging process may be terminated at a lower voltage. In the example described above, this might be 12 V, instead of 13.5 V. While this technique may prevent damage to the individual supercapacitors, it has the disadvantage that the maximum possible amount of energy storage is not achieved. The stored energy is given by the following equation:W=½CV2  (2)where W is the energy, in joules, C is the capacitance, in farads, and V is the capacitor voltage, in volts.
While using a single charger to provide a charging current to the series string, a balancing circuit may be added to force the voltages on all of the supercapacitors to be equal. This technique is widely used, and numerous circuits are available to provide this capability. Typically, these circuits measure the individual supercapacitor voltages, and turn on external shunt resistors to discharge the supercapacitors with the highest voltages, eventually forcing all of the voltages to become equal to the lowest voltage.
Instead of a single charger for the entire bank, individual isolated, or floating, chargers may be used for each supercapacitor. Each charger charges one supercapacitor to its full voltage, which is 2.7 V.
Supercapacitors are similar in many respects to lithium-ion battery cells, although lithium-ion cells are used to a much greater extent. A great deal of technology has been developed specifically for charging and balancing lithium-ion batteries, including low-cost integrated circuits. Like supercapacitors, lithium-ion cells are also “not forgiving” with regard to being overcharged, with a rated voltage near 4.0 V, rather than 2.7 V. Furthermore, lithium-ion cells have an additional problem, which is they must not go into deep discharge, which is the condition of being discharged to near zero volts. In general, if a lithium-ion cell is overcharged (>4V), it may go into thermal runaway and catch fire or explode. If it is subjected to deep discharge (near 0 V), it may short circuit, and cannot be recharged, and therefore becomes unusable.
Method and devices for the balancing of lithium-ion cells are described in U.S. Pat. No. 8,058,844 entitled “Low Power Battery System” and issued Nov. 15, 2011, incorporated herein by reference in its entirety. The primary limitation is that it does not work for very low voltages, e.g. less than 1.5 V or so. That is acceptable for lithium-ion cells, but not for supercapacitors, which can be fully discharged, down to 0 V.